Fix malloc_utils features (sysmalloc) (#7770)

Follow-up to:

- https://github.com/sigp/lighthouse/pull/7764

The `heaptrack` feature added in my previous PR was ineffective, because the jemalloc feature was turned on by the Linux target-specific dependency.

This PR tweaks the features such that:

- The jemalloc feature is just used to control whether jemalloc is compiled in. It is enabled on Linux by the target-specific dependency (see `lighthouse/Cargo.toml`), and completely disabled on Windows.
- If the `sysmalloc` feature is set on Linux then it overrides jemalloc when selecting an allocator, _even if_ the jemalloc feature is enabled (and the jemalloc dep was compiled).

book/src/installation_source.md

@@ -166,8 +166,8 @@ Commonly used features include:
 - `slasher-lmdb`: support for the LMDB slasher backend. Enabled by default.
 - `slasher-mdbx`: support for the MDBX slasher backend.
 - `beacon-node-leveldb`: support for the leveldb backend. Enabled by default.
-- `jemalloc`: use [`jemalloc`][jemalloc] to allocate memory. Enabled by default on Linux and macOS.
-  Not supported on Windows.
+- `sysmalloc`: use the system memory allocator rather than jemalloc. This is always enabled on
+  Windows.
 - `spec-minimal`: support for the minimal preset (useful for testing).
 
 Default features (e.g. `slasher-lmdb`, `beacon-node-leveldb`) may be opted out of using the `--no-default-features`
@@ -178,8 +178,6 @@ E.g.
 CARGO_INSTALL_EXTRA_FLAGS="--no-default-features" make
 ```
 
-[jemalloc]: https://jemalloc.net/
-
 ## Compilation Profiles
 
 You can customise the compiler settings used to compile Lighthouse via